PRINCIPLES OF TOXICOLOGY - Biology East Borneo

18.1 RISK ASSESSMENT BASICS 441For example, a risk assessment for a contaminated site might include consideration of workers at thesite, occasional trespassers or visitors to the site, or individuals who live at the site if the land is (ormight become) used for residential purposes. If residential land use is contemplated, risks are oftencalculated separately for children and adults, since they may be exposed to different extents andtherefore have different risks. Depending on the goals of the risk assessment, risks may be calculatedfor one or several populations of interest.Many chemicals move readily in the environment, from one medium to another. Thus, a chemicalspilled on the ground can volatilize into the air, migrate to groundwater and contaminate a drinkingwater supply, or be carried with surface water runoff to a nearby stream or lake. Risk assessments haveto be cognizant of environmental movement of chemicals, and the fact that an individual can be exposedto chemicals by a variety of pathways. In formulating the risk problem, the risk assessor must determinewhich of many possible pathways are complete; that is, which pathways will result in movement ofchemicals to a point where contact with an individual will occur. Each complete pathway provides theopportunity for the individual to receive a dose of the chemical, and should be considered in somefashion in the risk assessment. Incomplete exposure pathways—those that do not result in an individualcoming in contact with contaminated environmental media (e.g., air, water, soil)—can be ignored,since they offer no possibility of receiving a dose of chemical and therefore pose no risk.Risk assessments can vary considerably in the extent to which information on environmental fateof contaminants is included in the analysis. Some risk assessments, for example, have attempted toaddress risks posed by chemicals released to the air in incinerator emissions, and subsequentlydeposited on the ground where they are taken up by forage crops that are consumed by dairy cattle.Consumption of meat or milk from these cattle was regarded as a complete exposure pathway fromthe incinerator to a human receptor. As the thoroughness of the risk assessment increases, so does thecomplexity. As a practical matter, complete exposure pathways that are thought to be minor contributorsto total exposure and risk are often acknowledged but not included in the calculation of risk to makethe analysis more manageable.Often, exposure can lead to uptake of a chemical by more than one route. For example, contaminants insoil can enter the body through dermal absorption, accidental ingestion of small amounts of soil, or inhalationof contaminants volatilized from soil or adherent to small dust particles. Consequently, the manner ofanticipated exposure is important to consider, as it will dictate the routes of exposure (i.e., inhalation,dermal contact, or ingestion) that need to be included in the risk assessment for each exposure scenario.As discussed in the following section contrasting human health and ecological risk assessment,problem formulation is more challenging when conducting ecological risk assessments. Instead of onespecies, there are several to consider. Also, the exposure pathway analysis is more complicated, at leastin part because some of the species of interest consume other species of interest, thereby acquiringtheir body burden of chemical. Unlike human health risk assessments, where protection of individualsagainst any serious health impact is nearly always the objective, goals for ecological risk assessmentsare often at the population, or even ecosystem, level rather than focusing on individual plants andanimals. Consequently, development of assessment and measurement endpoints consistent with thegoals of the ecological risk assessment is essential in problem formulation for these kinds of analyses.Human Health versus Ecological Risk Assessments: Fundamental DifferencesEcological risk assessments are defined as those that address species other than humans, namely, plantand wildlife populations. Historically, the risk assessment process has focused primarily on addressingpotential adverse effects to exposed human populations, and the development of well-defined methodsfor human health risk assessment preceded those for ecological risk assessment. However, increasingconcern for ecological impacts of chemical contamination has led to a “catching up” in risk assessmentmethodology. While detailed methods for both human health and ecological risk assessment are nowin place, they aren’t identical. The conceptual basis may be similar, including some form of hazardidentification, exposure assessment, dose–response assessment, and risk characterization. However,